Astronomical Telescope Question: Why is telephoto suitable for looking at planets and short focus suitable for looking at depth?

This concept is aimed at lower-priced common achromatic (referring to universal elimination) or popular cattle inversion. Not suitable for apochromatic or anti-apochromatic products with top performance.

In order to control the cost, the optical accuracy of the main mirror is greatly reduced. Low precision means low image quality, and the degree of image quality degradation is related to the focal length of the main mirror. The shorter the focal length (or the greater the focal ratio), the more serious this effect is. That is to say, for two primary mirrors with the same aperture and optical accuracy, the imaging quality of telephoto lens (referring to the imaging quality directly on the focal plane of the primary mirror) is better than that of short-focus lens. This means that OTA is equipped with a telephoto primary mirror with the same aperture, allowing a larger magnification.

Let's go back and talk about the difference between planets as observation targets and deep space objects. Planets are characterized by small observation surface and high brightness, allowing and requiring high magnification in order to observe or photograph surface details. So telephoto lens is more suitable for observing or shooting planets. However, most celestial bodies in deep space have the characteristics of large observation surface, dispersion and low brightness, which require low magnification and large field of view observation. These characteristics correspond to the low image quality of short shots.

In fact, a telephoto lens can be equipped with a zoom system to obtain a high-quality short focal length, and a short focal length lens can also be equipped with a zoom system to obtain a low-quality long focal length. Therefore, the so-called advantages of a short lens are limited to its portability or portability. If you have enough space, the telephoto lens will get better results when observing deep space objects at low magnification.